1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as copying machines, facsimiles, and printers using an electrophotographic system.
More particularly, the present invention relates to an electrophotographic image forming apparatus comprising: an exposure section using blue laser having a short wavelength as writing exposure light; a discharge section using red LED having a long wavelength as discharge light; and an electrophotographic photoconductor containing a charge generation material having absorption ranges for both the wavelengths.
2. Description of the Related Art
Image formation of an electrophotographic system is performed by repeating steps of charging, exposing, developing, transferring, cleaning, and discharging around a photoconductor.
In recent years, concerning image quality, demands for higher printing resolution have increased combined with demands for higher definition and colorization.
In order to obtain higher printing resolution, it is necessary to make the diameter of an exposure spot smaller. And, in order to make the diameter of the exposure spot smaller, it is effective to shorten the oscillation wavelength of its light source.
For example, when a short-wavelength laser having an oscillation wavelength that is approximately half that of a conventional laser in a near-infrared region is used as the light source, the spot diameter of the laser beam on a photosensitive layer can be theoretically decreased by almost half as indicated by the following formula (1):d∝(π/4)(λf/D)  (1)wherein d is a spot diameter on the photosensitive layer, π is the circular constant, λ is a wave length of the laser beam, f is a focal length of an f θ lens, and D is a diameter of the lens.
Thus, shortening of the oscillation wavelength of the exposure light is very advantageous to increase of the writing density for latent images, that is, increase of the resolution.
Meanwhile, since the energy of individual photons increases in inverse proportion to wavelength, blue light having a short wavelength in a near-ultraviolet region is more likely to chemically change substances by photo-deterioration compared with red light having a long wavelength as the substances are repeatedly exposed to the short-wavelength light for or over a long period of time.
That is, substances (charge generation material and/or charge transfer material included in a photoconductor in the case of an electrophotographic image forming apparatus) exposed to light having a short wavelength over a long period of time are subjected to photo-deterioration.
Japanese Unexamined Patent Publication No. 2005-181991 discloses use of discharge light having a wavelength longer than that of exposure light. In Japanese Unexamined Patent Publication No. 2005-181991, however, the relationship in the wavelength between the discharge light and the exposure light is within a range of 380 nm to 520 nm, and blue light having a wavelength of 520 nm or less is still used as the discharge light. That is, the photoconductor is still subjected to photo-deterioration as used over a period of time.
Photoconductors are irradiated with light in an exposing step by an exposure means and in a discharging step by a discharge means.
The discharging step is to eliminate unevenness of charges remaining on the surface of the photoconductor after a transferring step and a cleaning step by applying light to the whole area of the photoconductor, and is necessary to regain an evenly charged state in a subsequent charging step.
Generally, the amount of discharge light is approximately 3 times to 5 times the amount of exposure light.
The exposure light is applied only to an image region, more specifically to an image part of the image region after being modulated to be in an amount according to each image density. On the other hand, the discharge light is applied to the whole region in a constant amount before the charging step.
That is, in a series of image formation processes of charge, exposure, development, transfer, cleaning, and discharge, the exposure means applies light in an amount according to the image density to the photoconductor only in part corresponding to the size of the image, more specifically in part where the image exists.
On the other hand, the discharge means necessarily applies light in an amount 3 times to 5 times the maximum amount of the exposure light to the whole region before the charging step in the above-described series of image formation processes.
That is, most of the light to be applied to the photoconductor is discharge light.
Generally, in image forming apparatuses for high printing resolution in which blue light having a short wavelength is used for exposure writing, photoconductors having sensitivity in a region of the short wavelength are used, and therefore, in the discharging step, light having sensitivity in the wavelength region, that is, blue light is used for discharge as well. Accordingly, in image forming apparatuses that perform exposure with light having a short wavelength and discharge with light having a short wavelength, photoconductors are always exposed to light having a short wavelength, and the performance thereof deteriorates due to photo-deterioration as used over a period of time to cause degradation of images.